Download Project 2: Critical Thinking and Normality

Project 2: Critical Thinking and Normality
Group Members (order alphabetically): _________________________
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Project 2: Critical Thinking and Normality
Sometimes a researcher must decide whether a variable is normally distributed. There are several
ways to do this. One simple but very subjective method uses special graph paper, which is called
normal probability paper. For the distribution of systolic blood pressure readings given in Chapter
3 of the textbook, the following method can be used:
1.
Make a table, as shown.
Frequency
24
62
72
26
12
4
Boundaries
89.5–104.5
104.5–119.5
119.5–134.5
134.5–149.5
149.5–164.5
164.5–179.5
Cumulative frequency
Cumulative percent frequency
2. Compute the mean and the standard deviation.
Computed mean=_________,
Computed standard deviation=__________.
3. Find the cumulative frequencies for each class, and place the results in the third column.
4. Find the cumulative percents for each class by dividing each cumulative frequency by 200 (the
total frequencies) and multiplying by 100%.
(For the first class, it would be 24/200 *100% =12 %. Place these values in the last column.
Cumulative percent tells us that 12% of the observations are less than 104.5.)
5. Based on part 4, what percent of the observations are:
a. less than 89.5
b. less than 119.5
c. less than 179.5
6. Using the normal probability paper shown in Table 6–3, label the x axis with the class
boundaries as shown and plot the Cumulative percent percents.
7. If the points fall approximately in a straight line, it can be concluded that the distribution is
normal.
Do you feel that this distribution is approximately normal? Explain your answer.
8. To find an approximation of the mean or median using the plot, draw a horizontal line from the
50% point on the y axis over to the curve and then a vertical line down to the x axis. Compare
this approximation of the mean with the computed mean.
Approximation of the mean from the graph=____________
Computed mean (from part 2 above)=_________________
Compare the two values:
9. To find an approximation of the standard deviation, locate the values on the x axis that
correspond to the 16 and 84% values on the y axis. Subtract these two values and divide the
result by 2. Compare this approximate standard deviation to the computed standard deviation.
Approximate standard deviation from the graph=__________
Computed standard deviation (from part 2 above) =_________________
Compare the two values:
10. Explain why the method used in step 7 works.